# Ring Size Converter

> Converts a ring size between US/Canada numbers, UK/Ireland letters, EU/ISO 8653 (inner circumference in mm), Japanese numbers and the inner diameter or circumference in millimetres, using the US relation diameter = 11.63 mm + 0.8128 mm × size and the Wikipedia ring-size comparison table.

- Calculator id: `ring-size` · Category: Everyday Life (`everyday`) · Tool name: `convert_ring_size`
- Canonical page: https://tttkmbb.com/everyday/ring-size · This document: https://tttkmbb.com/everyday/ring-size.md · JSON definition: https://tttkmbb.com/everyday/ring-size.json

## Purpose

Converts a ring size between US/Canada numbers, UK/Ireland letters, EU/ISO 8653 (inner circumference in mm), Japanese numbers and the inner diameter or circumference in millimetres, using the US relation diameter = 11.63 mm + 0.8128 mm × size and the Wikipedia ring-size comparison table.

**Use when:** You know a ring's size in one system, or have measured a ring's inside diameter or a finger's circumference, and need the equivalent sizes in the other systems.

**Do not use when:** The finger has not been measured (measure its circumference at the knuckle with a strip of paper first), or you need a jeweller's fitting for a very wide band; sizes are geometric conversions, not fit advice.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `value` | string |  | required | Size in the chosen system: a number (7, 7.5, 7¼, 54, 17.3) or, for UK, a letter with an optional half (N, N½, N1/2). |
| `system` | enum: us \| uk \| eu_iso \| japan \| diameter_mm \| circumference_mm |  | required | System of the entered value. |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `diameter_mm` | number | mm | Inside diameter of the ring. |
| `circumference_mm` | number | mm | π × diameter; the ISO 8653 / EU size before rounding. |
| `us_size` | number |  | (diameter_mm − 11.63) / 0.8128, unrounded. |
| `us_size_nearest` | number |  | US size rounded to the nearest quarter size, the finest step rings are made in. |
| `uk_size` | string |  | Nearest letter or half letter (A–Z) of the comparison table. |
| `eu_iso_size` | integer |  | Inner circumference in mm rounded to a whole number. |
| `japan_size` | integer |  | 1 + 3 × (diameter_mm − 13) rounded: size 1 = 13.0 mm, one third of a millimetre per size (omitted below 13 mm). |
| `diameter_in` | number | in | Inside diameter in inches. |
| `note` | string |  | Rounding conventions and fit caveat. |

## Formula

`diameter_mm = 11.63 + 0.8128 × US = circumference_mm / π = 13 + (Japan − 1) / 3; circumference_mm = π × diameter_mm = EU/ISO 8653 size; UK: nearest entry of the letter table (A = 37.8, F = 44.2, N = 54.4, P = 57.0, Z = 69.7 mm circumference, ≈ 1.28 mm per letter, half sizes at the midpoints); Japan = 1 + 3 × (diameter_mm − 13)`

US whole sizes differ by 0.032 in (0.8128 mm) of inner diameter with US 3 = 14.07 mm; ISO 8653:2016 designates rings by inner circumference in mm; Japanese sizes step by 1/3 mm of diameter from 13 mm at size 1; UK letters follow the Wikipedia comparison table. Conversions are purely geometric, so unrounded values show where a ring falls between made sizes (¼ US, ½ UK letter, whole EU and Japanese sizes).

## Data Sources

- Ring size – US, UK, ISO 8653 and Japanese scales with comparison table (Wikipedia) — https://en.wikipedia.org/wiki/Ring_size (reference, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/ring-size?value=…&system=…`
- `POST https://tttkmbb.com/api/v1/calculate/ring-size` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/ring-size · OpenAPI operationId `convert_ring_size` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "ring-size", "inputs": {…}}`

## Example

- US 7: inputs `{"value":"7","system":"us"}` → `{"diameter_mm":17.32,"circumference_mm":54.4,"us_size":7,"us_size_nearest":7,"uk_size":"N","eu_iso_size":54,"japan_size":14}`
- 18 mm inner diameter: inputs `{"value":"18","system":"diameter_mm"}` → `{"diameter_mm":18,"circumference_mm":56.5,"us_size":7.84,"us_size_nearest":7.75,"uk_size":"O½","eu_iso_size":57,"japan_size":16}`

```
GET https://tttkmbb.com/api/v1/calculate/ring-size?value=7&system=us
```

## Limitations

The finger has not been measured (measure its circumference at the knuckle with a strip of paper first), or you need a jeweller's fitting for a very wide band; sizes are geometric conversions, not fit advice. US whole sizes differ by 0.032 in (0.8128 mm) of inner diameter with US 3 = 14.07 mm; ISO 8653:2016 designates rings by inner circumference in mm; Japanese sizes step by 1/3 mm of diameter from 13 mm at size 1; UK letters follow the Wikipedia comparison table. Conversions are purely geometric, so unrounded values show where a ring falls between made sizes (¼ US, ½ UK letter, whole EU and Japanese sizes). All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**How do I measure my finger?**

Wrap a strip of paper snugly around the base of the finger, mark where it overlaps and measure the length in mm; enter it with system circumference_mm. Measure at the end of the day when fingers are largest, and make sure the ring passes the knuckle.

**Does band width change the size?**

Yes. Bands wider than about 6 mm feel tighter; jewellers usually recommend going up a quarter to a half US size for wide bands.

**Why does the UK letter not match some charts exactly?**

Charts differ by up to half a letter because British sizes were historically defined by diameter steps that do not align with US quarter sizes; this tool uses the Wikipedia comparison table.

## Related

- [Shoe Size Converter](https://tttkmbb.com/everyday/shoe-size.md) — Shoe sizes between US, UK, EU and cm.
- [Circle Calculator](https://tttkmbb.com/geometry/circle.md) — Circumference and diameter of any circle.